The present invention relates to a method for preparing CdTe.
The semiconductor CdTe has long been a subject of development interest in the semiconductor art. Its primary application today is as a lattice-matched substrate for HgCdTe infrared imaging devices. For such substrate applications, it is highly desirable to have reasonably large monocrystalline portions of CdTe, so that large epitaxial device regions without grain boundaries can be grown thereon. It is also desirable that the CdTe substrates not include harmful impurities which can diffuse into the HgCdTe active device layers.
One of the first obstacles which has been encountered in the growth of single crystal CdTe is the propensity of CdTe, when compounded from its elements, to adhere tenaciously to fused silica ampoules. The adhesion is so strong that, during cool down from the CdTe melting point (1100 C.) the quartz will fracture into tiny fragments, and it may still be necessary to separate the CdTe from the quartz by force. The usual solution to this problem is to cover the quartz with a carbon coating to effectively prevent direct contact between the quartz and the CdTe.
However, it is undesirable to compound the CdTe in contact with a carbon lining, since the effects of carbon doping in CdTe (and in HgCdTe device structures, which are commonly fabricated on CdTe substrates) are not believed to be beneficial.
Thus it is an object of the present invention to provide a method for compounding CdTe which does not require that the CdTe be compounded in the direct pressure of carbon. If CdTe adheres strongly to the quartz container, so that it must be removed by violent pounding, additional impurities are likely to be introduced by the separation process. Moreover, the whole object of growing large single-crystal masses of CdTe is frustrated if those masses are broken up during the process of removal.
It is a further object of the present invention to provide a method for compounding CdTe, which does not require that CdTe be subjected to damage when removed from quartz containers. The present invention solves this problem by completely avoiding the presence of cadmium oxides in the as-compounded CdTe. When cadmium oxides are not present, the CdTe does not stick to silica. Therefore, an unlined silica ampoule can used, and, after cooling, the CdTe ingot can merely be tipped out.
According to the present invention there is provided:
A method for compounding CdTe, comprising the steps of:
providing high-purity cadmium and Te;
distilling said high-purity Cd under a vacuum; and
compounding said Cd and Te to form a melt of CdTe;
wherein said Cd is maintained under vacuum continuously from said distillation step through said compounding step.